linux-kernel - Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Date:   Thu, 24 Feb 2022 13:00:14 +0000
From:   Aaron Tomlin <atomlin@...mlin.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Christoph Lameter <cl@...ux.com>, tglx@...utronix.de,
        mingo@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Phil Auld <pauld@...hat.com>
Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the
 idle tick was stopped too

On Thu 2022-02-24 09:27 -0300, Marcelo Tosatti wrote:
> But Aaron, vmstat_shepherd should be ensuring that per-CPU vmstat_update
> work are queued, if the per-CPU vmstat are out of sync.

Hi Marcelo,

Yes, I agree; albeit, as far as I understand, in the context of a nohz_full
CPU that has its scheduling-clock tick stopped, we cannot rely on any
deferred work.

The purpose of my patch was to prevent a nohz_full CPU from entering idle
state when CPU-specific vmstat data is non-zero.

> And:
> 
> static void
> trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
> {
>         if (!is_timers_nohz_active())
>                 return;
> 
>         /*
>          * TODO: This wants some optimizing similar to the code below, but we
>          * will do that when we switch from push to pull for deferrable timers.
>          */
>         if (timer->flags & TIMER_DEFERRABLE) {
>                 if (tick_nohz_full_cpu(base->cpu))
>                         wake_up_nohz_cpu(base->cpu);
>                 return;
>         }
> 
>  * @TIMER_DEFERRABLE: A deferrable timer will work normally when the
>  * system is busy, but will not cause a CPU to come out of idle just
>  * to service it; instead, the timer will be serviced when the CPU
>  * eventually wakes up with a subsequent non-deferrable timer.
> 
> You'd want that vmstat_update to execute regardless of whether there are 
> armed non-deferrable timers.
> 
> Should fix both 1 and 2 AFAICS.
> 

If I understand correctly, you are suggesting to switch to a non-deferred
timer for such work when the scheduling-clock tick is stopped? Indeed, it
would address both scenarios yet I'm not sure we'd want that due to the
performance impact which might be more than negligible.


Kind regards,

-- 
Aaron Tomlin